Messages in this thread |  | | | Date | Thu, 28 Apr 2011 12:32:32 +0200 | | From | Marek Szyprowski <> | | Subject | RE: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1 |
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Hello,
On Thursday, April 28, 2011 11:38 AM Russell King - ARM Linux wrote:
> > > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out > > > > that this is needed, and it currently is not implemented, with > > > > an outdated comment explaining why it used to not be possible > > > > to do it. > > > > > > dma_alloc_noncoherent is an entirely pointless API afaics. > > > > I was about to ask what the point is ... (what is the expected > > semantic ? Memory that is reachable but not necessarily cache > > coherent ?) > > As far as I can see, dma_alloc_noncoherent() should just be a wrapper > around the normal page allocation function. I don't see it ever needing > to do anything special - and the advantage of just being the normal > page allocation function is that its properties are well known and > architecture independent.
If there is IOMMU chip that supports pages larger than 4KiB then dma_alloc_noncoherent() might try to allocate such larger pages what will result in faster access to the buffer (lower iommu tlb miss ratio). For large buffers even 64KiB 'pages' gives a significant performance improvement.
Best regards -- Marek Szyprowski Samsung Poland R&D Center
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